The formation of carbonaccous particulates in a co-flow laminar diffusion flame has been studied using UV-visible spectroscopy and laser scattering/extinction techniques for measurements of volume fractions and particle sizes. Measurements were performed in a non-smoking ethylene-air flame at atmospheric pressure. UV-visible spectroscopy allowed the identification of two classes of particles: soot particles, which absorb light in the whole spectral range and nano-organic carbon particles (NOC) which are transparent to the visible radiation. The mean size of nano-organic carbon particles was estimated to be about 2-3 nm. This agreed with previous results obtained in rich premixed flames with equivalence ratios across the soot formation threshold. The experimental results indicate that nano-organic carbon particles are formed in the fuel side of the flame front closer to the flame centre line than soot particles and with a concentration level comparable to that of soot. The intermediate spatial location of these particles between the fuel- and soot-containing regions and their high concentration in flame suggest that soot formation is just the consequence of coagulation of NOC particles without a major role of surface growth in the soot loading process. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.